This invention relates to photocurable resins for stereolithography and compositions containing such resins.
Numerically controlled cutting process methods are used for the production of various kinds of models such as mold production models, profiling process models and die-sinking electric discharge process models as well as regular shape products. More recently, attention is being paid to the optical stereolithography whereby a photocurable resin or its composition is exposed to a radiation for hardening to form a desired stereo lithographed object. This invention relates to photocurable resins suitably adapted to such processes in stereolithography, and more particularly to photocurable resins for stereolithography containing unsaturated urethane and compositions containing such resins.
Examples of known photocurable resins for stereolithography containing unsaturated urethane include (1) those containing unsaturated urethane obtained from polyurethane diisocyanate and hydroxy alkyl acrylate (such as disclosed in Japanese Patent Publication Tokkai 2-145616), (2) those containing unsaturated urethane obtained from polyurethane diisocyanate and triol di(meth)acrylate (such as disclosed in .Japanese Patent Publications Tokkai 1-204915 and U.S. Pat. No. 4,879,402), (3) those containing unsaturated urethane obtained from polyurethane diisocyanate and tetraol triacrylate (such as disclosed in U.S. Pat. No. 4,608,409), (4) those containing unsaturated urethane obtained from polyurethane tri-.about.hexa-isocyanate and tri-ol di(meth)acrylate (such as disclosed in U.S. Pat. No. 4,879,402), (5) those containing unsaturated urethane obtained from triisocyanate and diol mono(meth)acrylate or tri-ol di(meth)acrylate (such as disclosed in U.S. Pat. No. 4,868,325), and (6) those containing unsaturated urethane obtained from diisocyanate and triol di(meth)acrylate (such as disclosed in U.S. Pat. No. 4,347,174). If an attempt is made to increase the double bond density within such unsaturated urethane so as to increase the crosslinking density of resulting stereolithographed objects, however, shrinkage becomes large at the time of photopolymerization reaction and the form precision of the resulting object becomes worse. If it is attempted, on the other hand, to reduce the double bond density within the unsaturated urethane so as to reduce the crosslinking density of the resulting stereolithographed object, thermal properties of such objects are adversely affected and, for example, their thermal deformation temperatures become lower.
In view of the above, there have been attempts to improve both the shrinkage at the time of reaction and the thermal properties by using unsaturated urethane of a kind having both polymerizable and long-chain hydrocarbon groups within the molecule, as disclosed, for example, in Japanese Patent Publications Tokkai 6-234818, 6-322039 and 6-287241. Although such prior art attempts could reduce the reaction shrinkage to a certain extent at the time of the photopolymerization reaction, it was not possible, as a matter of fact, to necessarily obtain stereolithographed objects with superior form precision. The reason for this is the existence of non-uniform stress inside the resulting objects caused by the material non-uniformity within the photocurable resin or its composition which has been used, non-uniformity in the photopolymerization reaction, and the shape of the stereolithographed object to be obtained. If the force of such stress becomes concentrated in a particular part of the object or a particular direction therein, there may result a deformation such as curvature, torsion and crush, as well as a fracture such as cracks and delamination. Thus, the form precision of such stereolithographed objects having such an internal stress is intrinsically inferior. In general, mechanical and thermal properties of stereolithographed objects obtained from prior art photocurable resins or their compositions, even with the aforementioned proposed attempts, are inferior. In particular, their toughness factor, defined as the product of the tensile strength and tensile elongation, is significantly inferior to the toughness factor of an ordinary thermoplastic resin such as the ABS resin commonly used as the material for plastic molding. Thus, the stereolithographed objects thus obtained can be used only for limited purposes.